The present invention relates to a control system for controlling a driving torque distribution between two drive wheels of a vehicle, and more specifically to a control system for controlling a clutch engagement force of a torque distribution clutch for varying a driving torque distribution between front and rear drive wheels, or between left and right drive wheels.
A Japanese Patent Provisional Publication No. 63-71428 (Application No. 61-217654) discloses a conventional differential limiting clutch control system for controlling a driving torque distribution between left and right drive wheels. This conventional control system is arranged to produce a control signal representing a command clutch engagement force T which is set equal to a sum of a desired clutch engagement force Ta, which is proportional to an amount of driver's operation applied to an accelerator and a desired clutch engagement force Tyg which is proportional to a lateral acceleration (or a centripetal acceleration) of the vehicle.
In order to prevent a spin of an inside wheel during a turn with a high lateral acceleration and to improve an accelerating ability during a turn in this conventional control system, the clutch engagement force T must be increased, to increase the differential limiting force, by increasing either of the desired clutch force Ta and the desired clutch force Tyg.
If, however, the desired clutch engagement force Tyg, which is responsive to the lateral acceleration, is increased, then the command clutch engagement force T becomes so high at the beginning of a cornering operation that the undesired understeering tendency is increased and the yawing response is adversely affected. Furthermore, an undesired phenomenon of tack-out takes place when the accelerator pedal is released during a high lateral acceleration turn of the vehicle. If on the other hand, the desired clutch engagement force Ta, which is responsive to the accelerating operation, is increased, then this conventional control system makes the differential limiting force high even during a straight ahead acceleration. Therefore, the possibility of a rear end swing becomes higher on a split friction road surface, and an actuator for controlling a fluid pressure is turned on and off too frequently.